Abstract

The evolution of the channel in photodissociation is examined using time-resolved negative-ion photoelectron imagingspectroscopy. The 388 nm photodetachmentimages obtained at variable delays following 388 nm excitation reveal the transformation of the excess electron from that belonging to an excited trihalide anion to that occupying an atomic orbital localized on the fragment. With increasing pump-probe delay, the corresponding photoelectron band narrows on a ∼300 fs time scale. This trend is attributed to the localization of the excess-electron wave function on the atomic-anion fragment and the establishment of the fragment’s electronic identity. The corresponding band position drifts towards larger electron kinetic energies on a significantly longer, ∼1 ps, time scale. The gradual spectral shift is attributed to exit-channel interactions affecting the photodetachment energetics, as well as the photoelectron anisotropy. The time-resolvedangular distributions are analyzed and found consistent with the formation of the asymptotic fragment.